In airplanes and trains which generate considerable noise, when information is offered by voice service to the users, problems arise with noise generated at the seats.
In use of an inside space whose boundary is formed by a continuous wall as in an airplane or a train, the place used is a kind of closed structure, and if noise sources exist inside and outside the place used, it will cause a noise environment to be fixed for the users. As a result, although it depends upon the level of noise, the noise exerts physical and mental pressure upon the user, worsening the comfortability of the place used. Particularly, in the case of offering a service to the passengers in the cabin of an airplane, worsening of the comfortability will give serious hindrance to the quality of service operation.
Particularly, in the case of an airplane, main noise sources include noise generated from machines such as propellers and engines for producing thrust forces of the airplane, and sound of air current generated as the airplane body moves in the air such as sound generated by end portions or both wings of the airplane flying through the air. In this case, inside noise makes the passengers feel uncomfortable and also causes hindrance to the voice service. Accordingly, the situation is strongly desired to be improved.
On the other hand, as a measure for reducing noise in a closed room, conventionally, a method using a passive attenuation means is commonly employed. In the conventional method, a sound insulation material having acoustic absorptivity such as partition wall material or absorptive material is arranged between the closed structure and the noise generating source. A high-density partition wall material is used as partition wall material, and a sound absorbing sheet is used as absorptive material. A material having acoustic absorptivity is generally high-density, and such a high-density material increases in weight. As the weight increases, it causes the flying fuel to increase, and the flying distance to shorten. Accordingly, it results in worsening of the economy and function as an airplane. Also, as a structural material, lowering of the strength such as being easy to be damaged and worsening of the design function such as quality of the touch cannot be ignored.
In order to solve the problem involved in the measure for noise by using a passive attenuation means, as a method of reducing noise by using an active attenuation means, a conventional method of generating sound wave having a phase opposite to the phase of noise is generally executed. By using this method, it is possible to lower the noise level at the noise source or in the vicinity thereof in order to prevent the noise from being propagated to regions required to be reduced in noise. As a specific example, an audio-erasing device has been proposed, comprising a microphone for detecting sound generated from noise sources, a controller which amplifies the electric signal input from the microphone and reverses the phase, and a speaker which converts the electric signal input from the controller to sound and transmits the sound (e.g. refer to Patent document 1).
Also, recently examined is a method of executing measures for noise from the viewpoint of improving the comfortability at passenger seats, which is based on a method of reducing noise by using the above-mentioned active attenuation means. For example, those proposed include a method of disposing a noise reducing device in each seat and installing a speaker, microphone and controller in the vicinity of the seat, and a method of increasing the noise reducing effect for the user by disposing a plurality of speakers and microphones in the vicinity of the user at the seat (e.g. refer to Patent documents 2 to 3).
However, for effectively reducing noise by using the above-mentioned method, there arise such problems that it is necessary to increase the number of microphones installed, the cost of the equipment is increased, and the calculated load for noise reduction becomes greater.    Patent document 1 Unexamined Japanese Patent Publication H1-270489    Patent document 2 Unexamined Japanese Patent Publication H5-289676    Patent document 3 Unexamined Japanese Patent Publication H5-281980